This invention pertains to inkjet inks, in particular to smear resistant inkjet inks, and even more particularly to smear resistant pigmented inkjet inks containing polyurethane dispersions.
Inkjet recording is a printing method wherein droplets of ink are ejected through fine nozzles to form letters or figures on the surface of recording media. Inks used in such recording are subject to rigorous demands including, for example, good dispersion stability, ejection stability, and good fixation to media.
Both dyes and pigments have been used as colorants for inkjet inks. While dyes typically offer superior color properties compared to pigments, they tend to be fade quickly and are more prone to rub off. Inks comprising pigments dispersed in aqueous media are advantageously superior to inks using water-soluble dyes in water-fastness and light-fastness of printed images.
Pigments suitable for aqueous inkjet inks are in general well-known in the art. Traditionally, pigments were stabilized by dispersing agents, such as polymeric dispersants or surfactants, to produce a stable dispersion of the pigment in the vehicle. More recently though, so-called “self-dispersible” or “self-dispersing” pigments (hereafter “SDP”) have been developed. As the name would imply, SDPs are dispersible in water without dispersants.
SDPs are often advantageous over traditional dispersant stabilized pigments from the standpoint of greater stability and lower viscosity at the same pigment loading. This can provide greater formulation latitude in final ink.
SDPs, and particularly self-dispersing carbon black pigments, are disclosed in, for example, U.S. Pat. No. 2,439,442, U.S. Pat. No. 3,023,118, U.S. Pat. No. 3,279,935 and U.S. Pat. No. 3,347,632. Additional disclosures of SDPs, methods of making SDPs and/or aqueous ink jet inks formulated with SDP's can be found in, for example, U.S. Pat. No. 5,554,739, U.S. Pat. No. 5,571,311, U.S. Pat. No. 5,609,671, U.S. Pat. No. 5,672,198, U.S. Pat. No. 5,698,016, U.S. Pat. No. 5,707,432, U.S. Pat. No. 5,718,746, U.S. Pat. No. 5,747,562, U.S. Pat. No. 5,749,950, U.S. Pat. No. 5,803,959, U.S. Pat. No. 5,837,045, U.S. Pat. No. 5,846,307, U.S. Pat. No. 5,851,280, U.S. Pat. No. 5,861,447, U.S. Pat. No. 5,885,335, U.S. Pat. No. 5,895,522, U.S. Pat. No. 5,922,118, U.S. Pat. No. 5,928,419, U.S. Pat. No. 5,976,233, U.S. Pat. No. 6,057,384, U.S. Pat. No. 6,099,632, U.S. Pat. No. 6,123,759, U.S. Pat. No. 6,153,001, U.S. Pat. No. 6,221,141, U.S. Pat. No. 6,221,142, U.S. Pat. No. 6,221,143, U.S. Pat. No. 6,281,267, U.S. Pat. No. 6,329,446, US2001/0035110, EP-A-1114851, EP-A-1158030, WO01/10963, WO01/25340 and WO01/94476.
All of the above disclosures are incorporated by reference herein for all purposes as if fully set forth.
When printing on porous media such as paper, penetrating agents are often added to shorten the drying time of printed images and, in addition, to form large pixels using a small amount of ink.
Although advantageous in some regards, penetrating pigment-based ink compositions can cause the color development to be somewhat poor when ink is printed on plain paper. Increasing the content of the pigment can improve color development but it generally also increases the viscosity of ink and thus is often unfavorable for the ejection stability of ink. However, the beneficial properties of SDPs allow these pigments to be loaded to higher levels with less impact on viscosity. Thus a penetrating ink having excellent color development is still possible by using SDPs.
Despite these potential advantages to the use of SDPs, inks formulated with SDPs tend to be somewhat poor in fixation on recording media, in particular plain paper.
Water resistance (fastness) is one type of fixation. As used herein, “water resistance” refers to the resistance of a print to blurring when contacted with drops of water. A printed page should not become blurred or illegible if wetted.
Smear resistance (fastness) is another type fixation. As used herein, “smear resistance” refers to the resistance of a print to blurring when stroked with a highlighter marker.
A printed page, especially text, should not become blurred or illegible if subjected to a highlighter. Smear resistance has been particularly difficult to achieve with an inkjet ink. An ink that is water-fast is not necessarily smear-fast.
Previously incorporated EP-A-1114851 demonstrates (Comparative Example 2 in Table 3) the problem of poor smear resistance in an SDP inkjet ink (therein referred to as rubbing/scratching resistance). There is taught the combination of SDP and dispersant stabilized pigment to improve image properties.
Previously incorporated EP-A-1158030 likewise demonstrates (Example 9 in Table 1) the problem of poor smear resistance with SDP inkjet ink (therein is referred to as high-lighter resistance).
Previously incorporated U.S. Pat. No. 6,329,446 discloses an inkjet ink with SDP and an acrylic emulsion for improved optical density and smear resistance.
Previously incorporated U.S. Pat. No. 6,057,384 discloses an inkjet ink with SDP and a core/shell acrylic latex to provide smear-fast prints.
Still, there is need for dispersion stable, ejection stable inkjet inks comprising an SDP which can print with good color on plain paper and which are water- and smear-fast.